won't power up!

I have a Dell inspiron 530.It will not power up The blue light comes on and the fans are running ,nothing else .When it is turned on the monitor says that it is in power safe mode but goes off again .The computer is about three years old

This is most likely a Power Supply (PSU) failure. It's producing power, just not enough to power the entire computer. Out of five Instpiron 530/531 computers I purchased I replaced three PSUs, all with the very same symptoms. Any ATX PSU will fit in the Inspiron 530. I prefer modular as it cuts down on the cable clutter.